A mother's exposure to airborne pollutants at work during her pregnancy may increase the likelihood that her unborn child will later develop asthma, a Danish study said.

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The review of registry data on 45,658 seven-year-old children and their mothers found that 18.6 percent of children of mothers who were exposed to low-molecular-weight particles at work during pregnancy developed asthma, compared to 16.1 percent of the general population.

"This is the first large-scale study which has shown an association between maternal exposures during work and asthma in children," said study leader Berit Christensen, at the School of Public Health in Denmark, in a statement.

For the study, which was presented at the European Respiratory Society's recent annual congress in Amsterdam, Christensen and colleagues used mothers' job titles to estimate their exposure to workplace pollutants, with categories for either low- or high-molecular-weight particles, mixed, farmers, "unclassifiable" and students, as well as a reference group of office workers for comparison.

After adjusting for age, body mass index, allergy and hypersensitivities, smoking, medication and pets, there was a slightly higher risk -- about 11 percent -- for asthma in children when their pregnant mothers were exposed to particles of both low molecular weight and high molecular weight.

The researchers found no asthma associations in the other exposure groups.